Electrochemical measurements are widely used to determine the concentration of specific substances in fluids. These devices, referred to as ion-selective electrodes (ISEs), can be employed in a wide variety of potentiometric ion determinations, including, for example, the activity of fluoride ion in drinking water, the pH of process streams, and the determination of electrolytes in serum.
In the health care field, and in particularly in the area of clinical diagnostics, ISEs are commonly used to measure the activity or concentration of various ions and metabolites present in blood plasma, serum and other biological fluids. For example, ISEs are typically used to determine Na.sup.+, Ca.sup.++, Mg.sup.++, K.sup.+, Cl.sup.-, Li.sup.+, pH, and carbon dioxide content in such fluids.
In general, conventional ISEs contain an ion-selective membrane, an internal filling solution or electrolyte, and an internal reference electrode. An external reference electrode used in conjunction with the ISE is typically a metal/metal halide electrode such as Ag/AgCl.
Conventional ISEs are typically bulky, expensive, difficult to clean and maintain, and tend to require an undesirably large volume of biological fluid. For these reasons, much attention has been directed towards developing more reliable ISEs of smaller size. These relatively small ISEs, referred to as ion-selective sensors or biosensors, can be inexpensively mass produced using techniques similar to these employed in the manufacture of electronic components, including for example, photolithography, screen printing, and ion-implantation. Ion-selective sensors and biosensors can be manufactured at much lower production cost than conventional ISEs, making it economically feasible to offer a single-use or limited-use disposable device, thereby eliminating the difficulty of cleaning and maintaining conventional ISEs. The reduced size of ion-selective sensors further serves to reduce the required volume of patient sample. Generally, a sensor can be either a miniature version of a conventional electrode or a device constructed using one or more of the above mentioned techniques. Maximum accuracy of the analytical or diagnostic result is obtained when the sensor responds only to the concentration or activity of the component of interest and has a response independent of the presence of interfering ions and/or underlying membrane matrix effects. The desired selectivity is often achieved by an ion-selective membrane containing an ion selective agent such as an ionophore. Generally, ion-selective membranes are formed from a heavily plasticized polymer matrix, such as polyvinyl chloride, which contains the ionophore selective for the ion of interest. For example, the ionophore valinomycin has been incorporated into a layer of membrane selective for potassium ions and trifluoroacetyl-p-butylbenzene or other trifluoroacetophenone derivatives has been used as ionophores selective for carbonate ions.
Determination of the concentration or activity of ionic species is achieved by measuring the EMF (electrical potential difference between the internal and external reference electrode, the electrodes being electrolytically connected by means of the sample solution at zero or near zero current flow) using a high impedance volt-meter. The reference electrode is electrolytically connected to the sample solution, typically by means of a salt bridge.
The accurate and rapid determination of total carbon dioxide species in physiological, industrial and environmental samples present a formidable challenge. Automated clinical analyzers are now routinely used for the determination of total carbon dioxide in biological samples using carbonate ion-selective electrodes or gas sensing electrodes configured as flow through detectors. However, none of the known carbonate selective membrane electrodes offer sufficient specificity or selectivity for unambiguous determination of carbonate ion. This is because ion-selective sensors utilizing carbonate selective ionophores are susceptible to interfering effects from comparatively large, hydrophobic ions such as perchlorate, gentisate, salicylate, p-amino salicylate and even larger species, such as heparin. Such salicylate ions are commonly present in physiological samples taken from patients treated with common analgesics.
Niedrach, U.S. Pat. No. 3,898,147, issued Aug. 5 1975, discloses a carbonate ion selective electrode having a hydrogen ion permeable membrane which consists of a hydrophobic elastomer. Measurement of pH of an aqueous electrolyte solution within the hydrogen ion permeable membrane serves as an indirect measurement of bicarbonate. However, this approach assumes absorption of carbon dioxide is the only source of hydrogen ions in the sample. Further, this system is susceptible to interferences from other endogenous acids.
Chapoteau, U.S. Pat. No. 4,810,351, issued Mar. 7, 1989, discloses a carbonate ion-selective membrane and electrode utilizing an ionophore consisting of an alkyl substituted fluoroacetophenone. High alkyl substitution of the ionophore is suggested to reduce drift in the electrode response in very fast flow situations. Further, the addition of a hydrophobic molecule to the ion selective membrane formulation is suggested to provide for the improved exclusion of interfering anions by repelling charged species from the solution phase. A C7-C9 quaternary ammonium surfactant is disclosed for binding the carbonate ions.
Kim et al., U.S. Pat. No. 4,272,328, issued Jun. 9, 1981, disclose an ion selective electrode multilayer analytical element which includes an ionophore containing membrane and a buffer zone in an amount sufficient to control the pH of the solution analyzed between about 7.5 to about 9.5. Under these conditions, it is suggested the element is comparatively less sensitive to the interfering effects of gentisate, salicylate and p-amino salicylate. Elimination of interferences from other large anions, such as, heparin and/or endogenous fatty acids is not mentioned.
Kim, U.S. Pat. No. 4,199,411, issued Apr. 22, 1980, discloses the structure of a halide ion-selective device having an overcoat made of a cellulose ester containing a polyol of from about 2 to about 6 hydroxyl groups. When added to the cellulose ester overcoat it was found the polyol "did not adversely affect the interference inhibiting characteristics of the cellulose ester layer". Hydrolysed cellulose acetate butyrate having from 6.4 to about 8.3 percent hydroxyl 0 groups is disclosed as one example of a cellulose ester. Kim does not mention or suggest the use of an asymmetric membrane or layers.
Anonymously, Research Disclosure 18730, November 1979 discloses the construction of halide ion-selective devices and the use of a halide-permeable overlayer comprising a cellulose ester to improve shelf life of the device. There is no mention or suggestion of an asymmetric membrane electrode.
Detwiler et al., U.S. Pat. No. 4,505,801, issued Mar. 19, 1985, disclose an overcoat layer for an ion-selective membrane, the overcoat layer having high selectivity for carbonate ions over potentially interfering ions, such as, salicylate, particularly at high salicylate concentrations. The disclosed overcoat layer has a discontinuous hydrophobic solvent dispersed within a continuous hydrophilic binder, a complexing agent for extracting oleophilic anions, a buffer which provides a pH in the range 7.5 to about 9.5 and a nucleating agent.
Ishizuka et al, U.S. Statutory Invention Registration No. H745, published Feb. 6, 1990, disclose the use of a buffered hydrophilic polymer binder layer laminated to a carbonate ion-selective layer for the purpose of eliminating or reducing interferences from salicylate, p-aminosalicylate and gentisate.
Cha et al., "Potentiometric Ion- and-Bio Selective Electrodes Based On Asymmetric Cellulose Acetate Membranes", Talanta, vol. 36, pp. 271-278, (1989) disclose a bioselective electrode consisting of a cellulose triacetate membrane having a hydrolyzed first layer fused to a second layer, the second layer containing a neutral carrier and plasticizer, and an enzyme covalently attached to the exposed hydrolyzed surface of the first layer. The potentiometric response of trifluoroacetyl-p-butylbenzene (TFABB)-doped asymmetric cellulose acetate membranes to carbonate and other anions is disclosed (FIG. 4 of Cha). It is further disclosed the "asymmetric modification (to form hydroxyl groups) had little or no effect on the response characteristics of the membrane. The response slope and selectivity (or lack of it with respect to salicylate) observed are essentially the same as those found for PVC based membranes previously reported by Greenberg and Meyerhoff."
The asymmetric membranes disclosed by Cha et al. were developed for biosensor applications. Cha et al. do not mention or suggest an enhanced selectivity process for potentiometric determinations. Further Cha et al. do not mention or suggest a process for potentiometric ion determinations in which an asymmetric membrane electrode provides for reduced interference from salicylate or such a process in which the activity of the ion of interest is determined kinetically in order to achieve enhanced selectivity, as opposed to statically (at equilibrium).